Serve hot or warm if possible, with a green salad for example.
Remarks
You can use whatever ham you prefer; I find that it's best with smoked ham, but it's a matter of taste. The same goes for the cheese. Gruyère, or even better Comté, will give you the best croque-monsieur.
yYu can cut the left-over crusts into small pieces and fry them in a little clarified butter or oil, to make small croutons which are great for adding to salads, for example.